Examination methods that require the use of electromagnetic waves, in particular short-wavelength light, are used in many ways in the examination of specimens. For example, it is necessary to inspect the results of producing a wafer, e.g., by inspecting layer thicknesses. A plurality of optical measurement arrangements, operating according to the principle of spectrophotometry or ellipsometry, are known from the existing art for the measurement of layer thicknesses. These arrangements allow both the layer thickness and the optical parameters of transparent layers to be determined very accurately. Short-wavelength light, i.e., light at a wavelength of less than 300 nm, is often used for such examinations. This light can derive, for example, from an excimer laser or from a special lamp. The short-wavelength light is often also made available by commercially available illumination sources in which the short wavelength is produced, on the basis of a solid-state laser, by frequency multiplication or by sum frequency mixing. In this process, the wavelength originally generated by the solid-state laser is modified by being conveyed to a device for frequency multiplication, in particular to a frequency doubler or a sum frequency mixer. Such devices make use of nonlinear effects that occur in certain crystals in response to high field strengths, thus generating the short-wavelength light. Examples of such commercially available illumination sources are e.g. the unit marketed by Coherent under the name AZURE 266, which generates a wavelength of 266 nm; or the unit marketed by the same company under the name INDIGU-DUV, which makes available a wavelength of 193 nm. These illumination sources are encapsulated in a housing as a complete assemblage, so that only the indicated nominal wavelength is therefore accessible and available at the exit of the illumination source.
As a result of stringent requirements in terms of reproducibility and accuracy, modern measurement systems for the semiconductor industry are nowadays usually installed in climate-controlled chambers. An important criterion is that, whenever possible, all heat sources that might negatively influence the measurement results are provided outside the climate-controlled chamber. In addition to electronics, these heat sources also include the illumination device, i.e., for example, the solid-state laser with the device for frequency multiplication. The illumination device is therefore positioned outside the chamber, and the light is coupled into a light-guiding fiber, for example, a quartz glass fiber. The light exit end of the fiber is guided to the measurement head in the climate-controlled chamber. An additional result of this is that vibration isolation of the illumination source can be dispensed with.
When short-wavelength light (at a wavelength shorter than 300 nm) is transmitted in a light guide, however, the relatively high absorptivity of the fiber means that the transmission length is very limited. In addition, the fiber ages relatively quickly as a result of, for example, color centers present in the fiber. The absorption of the short-wavelength light in the fiber thus rises even further.